Ultra High molecular weight polyethylene (UHMWPE) has been used as a bearing material for artificial joints such as hips and knees. Highly crosslinked UHMWPE is the current state-of-the-art for orthopedic bearing applications. Gamma or e-beam irradiation is the standard process to crosslink UHMWPE with practical irradiation dose about 3-10 Mrads. In this dose range, the wear rate is about 2.5 mm3/Mc at 9.5 Mrads and 20 mm3/Mc at 3.3 Mrads, respectively (See FIG. 22, U.S. Pat. No. 6,800,670 B2). Continuing increase of irradiation dose from 10 to 50 Mrads gradually drops the wear rate towards zero, but the material becomes too brittle for clinical use. It is desirable for UHMWPE that have wear rate about zero without increasing irradiation dose above 10 Mrads, thus mechanical properties does not suffer.
This invention relates to medical implants formed of a polymeric material such as ultra-high molecular weight polyethylene, with superior oxidation and wear resistance produced by an irradiation and annealing process followed by UV surface crosslinking. Alternatively the UHMWPE base material could be virgin UHMWPE or doped with an antioxidant such as anthocyanin.
Various polymer systems have been used for the preparation of artificial prostheses for biomedical use, particularly orthopedic applications. Among them, ultra-high molecular weight polyethylene is widely used for articulation surfaces in artificial knee, hip, and other joint replacements. Ultra-high molecular weight polyethylene (UHMWPE) has been defined as those linear polyethylenes which have a relative viscosity of 2.3 or greater at a solution concentration of 0.05% at 135° C. in decahydronaphthalene. The nominal weight−average molecular weight is at least 400,000 and up to 10,000,000 and usually from three to six million. The manufacturing process begins with the polymer being supplied as fine powder which is consolidated into various forms, such as rods and slabs, using ram extrusion or compression molding. Afterwards, the consolidated rods or slabs are machined into the final shape of the orthopedic implant components. Alternatively, the component can be produced by compression molding of the UHMWPE resin powder.
It has been recognized that regardless of the radiation type, the high energy beam causes generation of free radicals in polymers during radiation. It has also been recognized that the amount or number of free radicals generated is dependent upon the radiation dose received by the polymers and that the distribution of free radicals in the polymeric implant depends upon the geometry of the component, the type of polymer, the dose rate, and the type of radiation beam. The generation of free radicals can be described by the following reaction (which uses polyolefin and gamma ray irradiation for illustration):

If oxygen is present, primary free radicals r. will react with oxygen and the polymer according to the following reactions as described in “Radiation Effects on Polymers,” edited by Roger L. Clough and Shalaby W. Shalaby, published by American Chemical Society, Washington, D.C., 1991.
In the Presence of Oxygen

In radiation in air, primary free radicals r. will react with oxygen to form peroxyl free radicals r02., which then react with polyolefin (such as UHMWPE) to start the oxidative chain scission reactions (reactions 2 through 6). Through these reactions, material properties of the plastic, such as molecular weight, tensile and wear properties, are degraded.
It has been found that the hydroperoxides (rOOH and POOH) formed in reactions 3 and 5 will slowly break down as shown in reaction 7 to initiate post-radiation degradation. Reactions and 9 represent termination steps of free radicals to form ester or carbon-carbon cross-links. Depending on the type of polymer, the extent of reactions 8 and 9 in relation to reactions 2 through 7 may vary. For irradiated UHMWPE, a value of 0.3 for the ratio of chain scission to cross-linking has been obtained, indicating that even though cross-linking is a dominant mechanism, a significant amount of chain scission occurs in irradiated polyethylene.
By applying radiation in an inert atmosphere, since there is no oxidant present, the primary free radicals r. or secondary free radicals P. can only react with other neighboring free radicals to form carbon-carbon cross-links, according to reactions 10 through 12 below. If all the free radicals react through reactions 10 through 12, there will be no chain scission and there will be no molecular weight degradation. Furthermore, the extent of cross-linking is increased over the original polymer prior to irradiation. On the other hand, if not all the free radicals formed are combined through reactions 10, 11 and 12, then some free radicals will remain in the plastic component.
Out of Contact with Oxygen

It is recognized that the fewer the free radicals, the better the polymer retains its physical properties over time. The greater the number of free radicals, the greater the degree of molecular weight and polymer property degradation will occur. Applicant has discovered that the extent of completion of free radical cross-linking reactions is dependent on the reaction rates and the time period given for reaction to occur.
UHMWPE is commonly used to make prosthetic joints such as artificial hip joints. In recent years, it has been found that tissue necrosis and interface osteolysis may occur in response to UHMWPE wear debris. For example, wear of acetabular cups of UHMWPE in artificial hip joints may introduce microscopic wear particles into the surrounding tissues.
Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris may extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance.
It is known in the art that ultrahigh molecular weight polyethylene (UHMWPE) can be cross-linked by irradiation with high energy radiation, for example gamma or e-beam radiation, in an inert atmosphere or vacuum. Exposure of UHMWPE to gamma irradiation induces a number of free-radical reactions in the polymer. One of these is cross-linking. This cross-linking creates a 3-dimensional network in the polymer which renders it more resistant to adhesive wear in multiple directions. The free radicals formed upon irradiation of UHMWPE can also participate in oxidation which reduces the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement and a significant increase in wear rate. The free radicals are very long-lived (greater than eight years), so that oxidation continues over a very long period of time resulting in an increase in the wear rate as a result of oxidation over the life of the implant.
Sun et al. U.S. Pat. No. 5,414,049, the teachings of which are incorporated herein by reference, broadly discloses the use of radiation to form free radicals and heat to form cross-links between the free radicals prior to oxidation.
Hyun et al. U.S. Pat. No. 6,168,626 relates to a process for forming oriented UHMWPE materials for use in artificial joints by irradiating with low doses of high-energy radiation in an inert gas or vacuum to cross-link the material to a low degree, heating the irradiated material to a temperature at which compressive deformation is possible, preferably to a temperature near the melting point or higher, and performing compressive deformation followed by cooling and solidifying the material. The oriented UHMWPE materials have improved wear resistance. Medical implants may be machined from the oriented materials or molded directly during the compressive deformation step. The anisotropic nature of the oriented materials may render them susceptible to deformation after machining into implants.
Salovey et al. U.S. Pat. No. 6,228,900, the teachings of which are incorporated by reference, relates to a method for enhancing the wear-resistance of polymers, including UHMWPE, by cross-linking them via irradiation in the melt.
Saum et al. U.S. Pat. No. 6,316,158 relates to a process for treating UHMWPE using irradiation followed by thermally treating the polyethylene at a temperature greater than 150° C. to recombine cross-links and eliminate free radicals.
Sequential crosslinking is described in U.S. Pat. No. 7,517,919, the disclosure of which is incorporated by reference. An UHMWPE crosslinked three times as disclosed in U.S. Pat. No. 7,517,919 is designated herein as “X3”, a registered trademark of Stryker Corporation.
In the present invention ultraviolet (UV) radiation is applied to photocrosslink a UHMWPE bearing surface to generate an additional surface crosslinking layer on the already gamma or e-beam crosslinked bulk UHMWPE implant. The implant may be previously crosslinked at a dosage range between 1-10 Mrads or even higher. The surface layer thickness is controlled to a depth of about 100 micrometers. In this range, the surface layer can last at least 5 million cycles in a wear simulator test without showing measurable wear which can be seen by maintaining the original machining marks on the bearing surface.
Ultraviolet (UV) light crosslinking has been used for crosslinking polyethylene since the 1950's. In the past UV was used to crosslink polyethylene (PE) bulk material, fibers, and films by mixing a photo-initiator and PE resin, then consolidating the resin and crosslinking under UV irradiation (see for example U.S. Pat. No. 6,281,264 B1, Chen Y. L. et al., “Photocrosslinking of Polyethylene”, Journal Polymer Science, Polymer Chemistry Edition, 1989, Qu B. J., et al., “Photoinitiating Characteristics Of Benzophenone Derivatives As New Initiators In The Photocrosslinking Of Polyethylene, Polymer Engineering and Science, July 2001). On the other hand, PE degradation in air when exposed in sun light (low UV intensity) is a well-known phenomena. To prevent this, PE, an anti-UV additive is sometimes added to the polyethylene resin. However, none of the prior art disclosed using UV as a surface crosslinking method for a consolidated UHMWPE bearing, nor using a UV method to generate an additional crosslinking layer on a gamma or e-beam irradiated crosslinked UHMWPE bearing material for orthopedic applications such as in acetabular cup, glenoid bearings, tibial bearing surfaces, finger and elbow UHMWPE bearings. U.S. Pat. No. 6,165,220 relates to e-beam surface crosslinking. U.S. Patent Publication No. 20070270970 relates to polymeric bearings for use in the spine.
UV crosslinking of polyethylene is a reaction of carbon center free radicals that are generated by UV radiation. Mechanism of the UV crosslinking of polyethylene is briefly described below. When UV light radiates polyethylene that contains benzophenone, a photoinitiator, benzophenone absorbs UV energy and jumps to the excited triplet state. The benzophenone in the triplet excited state abstracts hydrogen from polyethylene to generate polyethylene carbon radicals. The formed PE carbon center radicals undergo free radical reaction to form crosslinking.

The quantum yield of the excited triplet state is very high for benzophenone and the triplet state is highly effective in hydrogen extraction. It is noted that there is no carbon-carbon bond breakage in the UV crosslinking process and the carbon radicals are generated solely by a carbon-hydrogen bond cleavage. Therefore, unlike gamma or e-beam crosslinking process, UV crosslinking does not result in a reduction of molecular weight.
Due to improved surface wear performance of surface photocrosslinked already bulk crosslinked UHMWPE, acetabular UHMWPE cups can be designed to be very thin (<3 mm) without the danger of wearing through. The current FDA standard is 6.0 mm thick UHMWPE. A thinner cup will maximize femoral head size. The bigger femoral head size can significantly decrease rate of dislocation, which is the number one cause of total hip replacement revision. Another benefit of the very thin UHMWPE bearing is to allow cobalt-chrome-on-UHMWPE for resurfacing hip applications. Current resurfacing bearing are typically cobalt chrome on cobalt chrome which may release metal ions which causes metal hypersensitivity in some patients.
Another benefit of the present invention is for knee tibial insert applications. One of the causes of knee revision is that the tibial implant is set on a soft bone bed. Overtime after implantation, the soft tibia bone allows the implant to sink to a lower position and the gap between femoral and tibia is increased, resulting in higher impact force accelerated wear of tibia insert. This invention allows the tibia insert becomes thinner, such as 3 mm, as compared to the normal 6 mm standard. The thinner tibia insert requires less of a bone cut. The bone in both femoral and tibia sides has a hardness gradient. The closer to the bearing portion of the knee joint, the harder the bone. Thus the thinner bone cut allows the knee implants to be set on the stronger bone.
Typically wear rates are higher than 2.5 mm/million cycles for 10 Mrads gamma irradiated bulk UHMWPE.
Thus additional UV surface photo-crosslinking performed on gamma ray or e-beam already crosslinked material unexpectedly produces lower wear than current crosslinked UHMWPE. The final wear performance is significantly improved as compared to the non-UV surface crosslinked surface made of highly crosslinked UHMWPE. The improved photocrosslink surface allows the use of alternate bearing material such as PAEK in general and PEEK in particular.